In NPL 1, a method for combining an electron energy loss spectroscopy (EELS) with a scanning transmission microscopy (STEM) is described.
The STEM is a device that observes a structure of a sample with a high spatial resolution by using an electron beam. In addition, the EELS can acquire an energy loss spectrum by interacting with the sample with high energy resolution by using an energy spectroscopy attached as an attachment device of the STEM. Furthermore, by selectively detecting the electron of specific energy, it is possible to acquire an energy filter image.
When a thin film sample is irradiated with the electron beam, the electron beam interacts the sample according to a type and a structure of elements configuring the sample. By selectively detecting an angle and energy of the transmitted electron beam, it is possible to acquire various types of information.
For example, an image formed by the electrons scattered at a low angle of several tens mrad or less or the electrons transmitted without scattering is called as a bright field image. On the other hand, information depending on the density of the sample is included in the electron beam scattered at a high angle, which is suitable for identifying constituent elements and is called as a dark field image. In a case where the dark field image is acquired by an annular detector, an optimum value presents in a scattering angle range to be detected. Although it depends on an acceleration voltage, for example, it is preferable to appropriately set an optimum value within a range of approximately 20 mrad to 300 mrad at 200 kV.
Similarly, there is also the optimum value within a scattering angle to be detected in the EELS. In NPL 2 (section 61), inelastically scattered electrons caused by plasmon excitation or the like spread out from the central beam such that the detection efficiency increases as the scattering angle to be detected increases and an analytical result with good S/N is given is described.
In PTL 1, in a TEM/STEM device to which the EELS is attached, by disposing an electron lens between an annular dark field electron detector and a bright field electron detector and by setting an object point of an EELS spectrometer as a virtual image, the mechanical incidence angle to the EELS spectrometer decreases without changing an acceptance angle to the annular dark field electron detector and an acceptance angle to the bright field electron detector is described.